


By Instinct

by Wolfish160



Series: By Chance [2]
Category: Shingeki no Kyojin | Attack on Titan
Genre: Anal Sex, Biting, Bottom Levi (Shingeki no Kyojin), M/M, Mating, Mermaid Isabel, Mermaid Mikasa, Mermaids, Mermen, Pups, Sirens, Top Eren Yeager, albino siren, heat - Freeform, merman eren, merman farlan, merman levi
Language: English
Status: Completed
Published: 2019-12-27
Updated: 2019-12-27
Packaged: 2021-02-26 00:00:12
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,391
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/21984040
Author URL: https://archiveofourown.org/users/Wolfish160/pseuds/Wolfish160
Summary: Sequel to By ChanceI don’t know how my life would’ve been if I never met Eren and this pack. I can’t imagine it would’ve been this perfect...well almost perfect.
Relationships: Furlan Church/Isabel Magnolia, Levi/Eren Yeager
Series: By Chance [2]
Series URL: https://archiveofourown.org/series/1582516
Comments: 6
Kudos: 124





	By Instinct

**Author's Note:**

> It's finally here!!!!! So sorry it took me so long to write this sequel needed to find that special inspiration to continue this story! I hope you'll enjoy it as I loved writing it!
> 
> Also wanted to hit myself for making the mer language be in bold letters as I often forgot to change it XD

**“You almost have it love, just try not to think about it too much.”** Eren chirped gently as he picked up on my annoyed mood. I should’ve met Eren’s pack an hour ago but I couldn’t figure out how to use this damn tail. I can’t recall how many times I’ve banged into the cave walls because I was going faster than intended. At the moment I had my arms crossed with my back to the merman refusing to move from my spot. One would call this sulking, but I’m not. Levi Ackerman doesn’t sulk! I rolled my eyes at myself, denying it makes it true. I felt webbed hands rest on my waist and a nuzzle against my shoulder making me relax a bit. I glance over at Eren who was giving me an encouraging smile.  **“You can do it Levi, think about when you pinned me after you woke up. It all falls back on you trusting your instincts and letting them flow through you. I’ve notice humans tend to overthink anything they’re unfamiliar with instead of going with their own instincts. That may be a way to survive on land, but down here in the sea it’s okay live through your instincts.”**

Eren’s words always seemed to calm me down one way or another. I nodded softly taking in a deep breath, through the new gills on the side of my neck, and thought about myself moving to the mouth of the cave. I watched calmly as my tail thrusted forward and pivoted me towards the entrance. When my tail relaxed, twitching occasionally to keep me balanced, I looked up to see I was at the mouth. Eren swam up next to me with a grin etched into his face causing a smile to spread across mine.  _ I want to rub my cheek against his, _ I thought. So I did as Eren told me and let my instincts flow through me. I circled my arms around the merman’s shoulders and lifted myself up to his eye level. Eren rose a surprised brow, but was nevertheless happy about my touch. I leaned forward tilting my head to the left as I began nuzzling the side of my cheek against his with a purr rumbling from my chest. Eren gave his own purr in response as he pulled me against his chest with the hands on my waist. 

**“I understand why you do this all the time now.”** I joked making Eren chuckle before pressing a kiss to my shoulder. 

**“Are you ready to meet them now?”** He asked and I nodded with a smile. 

**“It’s now or never.”** Eren returned my smile and took my hand in his as he lead me out of the cave. 

* * *

**“Isn’t his tail a little short?”** The raven haired mermaid deadpanned making my tail twitch in annoyance. Though she wasn’t wrong, as I observed the other mers around us I noticed theirs all rivaled mine in length. 

**“Mikasa.”** Eren hissed glancing at me nervously and I just crossed my arms shrugging.

**“Just means I can move faster.”** A small smirk curves into my lips making Mikasa roll her charcoal colored eyes. 

**“Anyways,”** Eren sighed softly as he turned to the mers surrounding us.  **“Everyone this is Levi, the human-now merman-I chose to be my mate and to lead the pack along with me.”** The brunet explained making my eyes widen slightly at his ending statement.  _ I’m going to lead with Eren?  _ The pack let out a few mild whistles and chirps welcoming me into their  _ family _ . Family, huh? It has been awhile since I’ve had a true family other than my best friend Hanji. But I lost Hanji when I borrowed her boat in the middle of the night and ended up stranded on an island. I hope she doesn’t miss me too much...or her boat. 

I’m knocked out of my thoughts when tiny arms latch themselves onto my newly formed tail. I looked down startled to find a child with sandy, blond hair and bright, blue eyes staring up at me. They held a childish grin on their face as they curled their teal colored tail around my own. I rose a brow making them giggle as they continued to stare. 

**“Sky! You don’t just hug mers you just met sweetheart.”** Someone scolds causing me to look up. I recognize them instantly as the blond merman who glared at me when I had apologized to the redheaded mermaid. Who just so happen to be next to him with a grin of her own as she watched the child latched onto my tail. 

**“Oh Farlan she’s just saying hi.”** She mused making Farlan roll his eyes. 

**“Yeah daddy! Levi’s our family now!”** The little mer beamed making a small smile come to my face. I reach my arm down and the girl wraps her tail around it similar to a seahorse would. I lift my arm up gently so that the child and I are at eye level. 

**“Are you the troublemaker Eren’s been telling me about?”** I ask making her giggle once again. 

**“Maybe!”** Sky chirped looking rather smug. 

**“Guess I better keep my eye on you.”**

**“Are you sure you want to make that promise? She’s quite the handful.”** Barked the redhead, who had swam over next to me. Sky stuck her tongue out and I chuckled quietly to myself.  **“Come one squirt, I’m sure Levi and Eren want to spend some alone time together.”** The mer chirped as she reached for Sky giving me a knowing look. I felt a slight warmth in my cheeks at the mers words. I felt Eren’s webbed fingers interlock with mine, pulling gently as he guided us away from the pack. 

* * *

The two of us tumbled into a large cave with our lips and tails locked together tightly. It was weird kissing underwater. Bubbles rose from our lips as they danced against each other making a disturbance in the water. Eren unlatched his tail from mine as he began to guide us up to the surface. I was surprised when our faces broke the surface of the sea instead of bumping into the cave ceiling. I blinked my eyes trying to get used to no longer being beneath the waves as I looked around. We were still in the cave, but this part of the cave wasn’t encased with the sea, but was flushed with greenery and sand. 

“It’s beautiful,” I told Eren in my human tongue as I admire the scenery before me. When I look back at the merman he’s smiling. 

**“I’ve been saving it for you- well for my future mate and that’s you.”** He informs me and I can’t help but smile back. I circle my arms around Eren’s neck as I leaned up to leave a soft kiss on his salty lips. That soft kiss quickly turns to something more erratic and rough as a new type of desire filled me. Of course I know what sexual desire feels like, but this felt different. To put it bluntly, it feels like I’ll die if I don’t get Eren’s cock in me. My whole body felt hot and for the life of me I couldn’t push myself away from my mate (not that I wanted to). It wasn’t until Eren pulled away to trail kisses and bites along my jaw and neck that I could ask what the fuck was going on. 

“E-Eren! What’s going on-hah! W-Why do I-I...feel like this?!” I cry out in my human tongue, gripping onto to my mate’s shoulders tightly. Eren circles his arms around my waist pulling me flush against his body while still nibbling at my neck. His tail then coils around mine securing me in my place in his arms. 

**“You mated me as a human but the mer inside you hasn’t been claimed yet, so your new instincts pushed you into a heat so you’d mate. I could smell it on you from the moment you woke up. I’m sure the pack did too, which is why they didn’t pester us for too long.”** Eren purred the words into my ear before leaning back to look at me. I knew my face was flushed red with desire, but Eren caught the little tinge of fear in my eyes.  **“There’s no need to be afraid my mate,”** Eren caressed my cheek as he says this.  **“Just allow your instincts in, just like you did when you learned to speak and swim. Nothing bad will come out of it, always trust them no matter what.”** He cooed instantly killing any fear of the unknown in me. I nodded to Eren who gave me a comforting smile. I closed my eyes, taking in a deep breath as I let every thought and desire take over my body, fully submitting to my instincts.

**“Eren~”** I purred as I nuzzled my face against my mate’s neck, occasionally nipping at the flesh. A low growl of approval rumbled in Eren’s chest making me shiver in anticipation. I felt Eren grind against me, rubbing his cock against something very sensitive. A gasp left my lips at the feeling, my gaze falling down between the two of us. Where my crotch once was, was a large slit that was slowly pulling apart to reveal the tip of my cock. Underneath the slit was a hole that was rather swollen and pulsing with need. That’s what Eren had rubbed against, and from what my instincts were telling me that’s where my mate’s cock belonged. I watched as Eren did it again making me jolt with pleasure causing my cock to fully slip out from the slit. 

Suddenly, I was pressed against a wall of sand making me look back up at my mate. Eren’s pupils swallowed the green in his eyes reminding me of when we first fucked on that lone island. I circle my arms around my mate pulling his lips closer to mine so I could capture them in a kiss full of sharp teeth and tongue. Eren continued to rut against me as we practically devoured one another. I moaned and purred into his mouth, curling my fingers in his long hair before pulling away to speak.

**“Mate me~”** I purr making my mate growl loudly sending shivers up my spine. He wastes no time positioning his cock against my rim, but before he pushes in he gives me a searing kiss. I go limp because of the passionate kiss but reel my head back breaking the it because Eren pushes into me. A moan escapes my lips as Eren pushes all the way in, pausing to make sure I’m alright. There’s a tiny trail of blood on his chin from wear I must have cut his lip when I jerked away. I lean forward licking the blood up and grind my lower half against Eren’s telling him to move. My mate purrs softly before letting his own desire completely take over him and begins to ram his hips into mine. Mantras of moans, purrs, and clicks leave my mouth as I’m being completely fucked into oblivion. The sensation of my cock rubbing against Eren’s stomach every time he hammered into me had me drooling. My claws dug into Eren’s back harshly from the pure pleasure rushing through my whole body. 

I thought when we fucked for the first time it was the best sex I’d ever had, but I couldn’t be more wrong. This right here was the best sex I’ll ever have because it’s so raw. We’re practically animals with no third world problems, making love to the creature we’re going to spend the rest of our lives with. I could truly feel all the love and passion Eren felt for me in every thrust, bite, and kiss. And I showed Eren with my own actions that I felt the same amount of love and passion for him. 

**“E-Eren~!”** I cry out as he hits that special bundle of nerves inside me, causing my back to arch off the sand.  **“Harder!”**

**“You take it so well my mate!”** Eren purrs before putting all his force into every thrust making me tip over the edge instantly. I feel my cum shoot into the water and against our stomachs. My mouth is open wide in a silently scream at the overwhelming feeling of pleasure. Eren grunts and growls as he continues to piston into me trying to reach his own release. His hips stutter as he gets closer and closer making me more of a mess.  **“Lean down!”** Eren growls through gritted teeth making me whimper in desire at the commanding voice. 

Once I lay my forehead on the other’s shoulder Eren gives one last thrust before biting into the nape of my neck sealing our bond once more. I completely slump against my mate as he pierces my flesh while pumping his seed into me. Growls and purrs leave Eren’s chest as his cum fills me to the brim leaving my instincts satisfied at last. He finally releases my neck and leans back to nuzzle every bit of my face and neck. 

**“Mine.”** He growls low and possessively as he continues to mark my skin with his scent. I smile blissfully pressing a sleepy kiss to his shoulder, relaxing completely into my mate’s warm embrace. A continuous purr rumbles in my throat that I make no effort to stop as it causes Eren to purr as well. 

**“I love you, Eren.”** I coo softly returning Eren’s nuzzle. Eren presses me closer to him, purring very loudly now as he cups my face. 

**“I love you too, my love.”** He coos back before leaning in to kiss me. 

* * *

**Several Months Later**

I never thought I’d say this but I’m so happy I wrecked Hanji’s boat and got stranded on that island. It all led me to Eren and the pack, my new family. Throughout the months I’ve warmed up to the pack and they’ve also warmed up to me. According to Eren I filled an empty spot that the pack desperately needed, let’s just say Eren got  _ very _ lucky afterwards. I’ve also become the best hunter of the pack, even better than Mikasa. My retort to her when he first met rang true, because of my smaller size I was able to move faster than any of the pack members. Despite taking her spot in the hunting ranks Mikasa and I grew a respect for one another and would do anything to protect the other. Isabel still tends to tease us of our earlier moments when we’d bicker back and forth like siblings. Speaking of that spitfire, we’ve become very close along with her mate Farlan. I don’t know what it was but the three of us just clicked like we had known each other our whole lives. Sky even calls me Uncle Levi these days. I don’t know how my life would’ve been if I never met Eren and this pack. I can’t imagine it would’ve been this perfect...well almost perfect. 

With Eren and I both being males, that means neither of us can bare pups. Even though Eren assured me that he was very aware and fine with this when he chose me to be his mate, I know how desperately he wants a pup. The way he looks at Sky says it all. The love and happiness in his eyes as he plays with the pup is so obvious, but so is the sad and want when her parents whisk her away for a nap. I know Eren loves Sky as if she were his own, but I know it’s not the same as having a pup that’s truly his to care for, to raise. And I can’t say I don’t want that too. 

**“God damnit.”** I curse as I swim near the ocean floor searching for the scent of a school of fish. Mikasa and I decided to split up to cover more ground as the fish were being rather sparse. Which is very unusual for this area, but the pack is hungry so we can’t dwell on that. I pause for a moment as I close my eyes and take in a deep breath through my gills looking for any scent that stands out. But of course I find nothing and let out a sigh causing bubbles to float to the surface. I turn to go another way, but a scent finally catches my nose. A scent of a _ siren _ . 

The fins along my spine flare up instantly, my teeth bare as I turn towards the scent. No siren comes into  _ our _ territory! A growl rumbled deep in my chest as I scan the area for the intruder. My ear twitches as a terrified whimper answers my threatening growl. Whoever it was must have been stupid because they moved causing bubbles to rise behind a rock. I use that speed of mine and round the rock letting out a growl even louder than before. 

Though the growl dies instantly as I’m met with beady, white terrified eyes staring back at me. Pressed against the rock is a pup that's as white as a perfect pearl. No literally, the pup has snowy white hair along with a shimmery white tail to match. I’ve never seen anything like it, it’s the most beautiful creature I’ve ever seen (other than Eren obviously). I take in a deep breath to confirm that this albino pup is the siren I smelled and it is. The pup whimpers once more making me realize how threatening I still am. Poor thing looks like it’s about to piss itself. I quickly relax my fins to hide the sharp spikes and cover my flesh tearing teeth with my lips. I lower myself slowly to the pup’s level to hopefully show him I’m not going to hurt him. 

**“Hey, it’s okay pup. I’m not going to hurt you.”** I chirp softly making his trembling ease slightly. 

**“B-But y-you growled at me l-like you were.”** He mumbled, his lip wobbling making me frown further.

**“I apologize for that. I smelled a siren, but I didn’t realize you were just a pup and I’d never hurt a pup.”** I tell him and I can see him slowly calming down from the scare.  **“Speaking of which, what are you doing here pup?”** This made him tense up all over again as he was filled with fear. 

**“I-I had to swim away b-because the others were g-going to h-hurt me!”** The pup cried breaking my heart. 

**“Why?”** I asked completely shocked, how could anyone hurt a child?!

**“I-I wouldn’t eat a h-human…”** He responds gripping onto his tail for comfort.  **“S-Said I had n-no choice because of ‘h-how pretty I-I am’.”** He adds. I was completely disgusted at the words that left the pup’s mouth. Did this also happen to Eren when he refused? That would scar any pup! 

**“I knew sirens could be horrible creatures, but that’s cruel.”** I growl softly shaking my head in disbelief.

**“D-Does that mean I-I will be c-cruel too?”** The pup whimpers and my face softens instantly. 

**“Of course not pup, you proved that by being brave enough to not eat a human. And I know this because my mate was also born as a siren, but refused just like you did.”** I tell the pup smiling slowly moving closer to the little albino. The pup smiled as well, all the fear gone now. Suddenly, a bundle of white was pressed against my chest causing me to grunt in surprise. The pup gripped tightly onto me with his dull little claws digging into my chest. I carefully wrapped an arm around the pup, holding him as I moved up from the sandy floor. My other hand cupped the back of his head as I nuzzled him gently letting out a comforting purr. As I did so a war was being fought in my head. 

_ I want to take him to the pack. But what if the sirens come for him? Then we’ll protect him. What if it turns into a war! Someone could die! But this feels right! Its what my… _

__ **_“There’s no need to be afraid my mate,”_ ** _ Eren caressed my cheek as he says this.  _ **_“Just allow your instincts in, just like you did when you learned to speak and swim. Nothing bad will come out of it, always trust them no matter what.”_ **

**** **“...Instincts are telling me to do.”** I mumble to myself gazing at the pup curled up in my arms.  **“Hey pup?”** He looks up at me with curious eyes.  **“Would you want to live with me and my pack? My mate and I can’t have pups and I know you’d be perfect for us.”** I bark smiling down at the boy, smiling harder when he started practically bouncing in my arms. 

**“Yes! Please!”** He chirped excitedly making me chuckle. 

**“Then it’s settled.”** I purr nuzzling the pup once again, getting one in return this time. 

**“What’s settled?”** Mikasa barks behind me making the pup and I tense up. I turned slowly to Mikasa with the pup peeking from my arms warily. Her eyes locked on him instantly and her gills flared as she caught his scent. 

**“He’s like Eren.”** I bark holding the pup tighter against me. Mikasa’s eyes widen as she looks at the pup and then back at me.  **“He swam away from the other sirens because they threatened to hurt him if he didn’t eat a human. I’ve decided that he’s going to be Eren and I’s pup now.”** I tell here not being able to hide my smile as I told the mer the last bit. I watch as Mikasa’s stone hard face melts and a small smile makes its way on her lips. 

**“Then I guess we better get the pup home shouldn’t we?”** She chirps making the pup perk up in excitement. 

**“Go back to the pack and tell Eren to come find me in our cave.”** I tell her and she nods sending me one last smile before swimming the other way. I start heading towards Eren and I’s private cave with the albino pup secure in my arms.  **“What’s your name pup?”** I ask and he looks up at me. 

**“Albus.”**

**“It suits you, my name is Levi.”** Albus tilts his head at me. 

**“I’ve never heard a name like that before...I don’t think I can even say it.”** He admits making me chuckle. 

**“Neither could Eren, must be because its a human name.”**

**“Why do you have a human name?”** Albus asks.

**“Because I was human before a few months ago.”** The pup’s eyes widen and I couldn’t help but grin. 

**“How?!”** He exclaimed completely amazed. 

**“Did the sirens ever tell you not to kiss a human?”**

**“That’s the first law.”** Albus informs me, his white eyebrows scrunched up before shooting up in realization.  **“You became a mer because you kissed your mate?”** I nod entering Eren and I’s cave with the pup pestering me with questions that made my amusement grow. 

**“I’ll answer all your questions later alright pup? You’ve got to meet my mate.”** I tell Albus making him tense up slightly. 

**“What if he doesn’t like me?”** He whimpers hiding his face in my chest. 

**“He’s going to love and adore you Albus, I promise.”** I coo nuzzling the pup gently making him purr. We stay like that for a few minutes before Eren’s scent enters the cave making me lift my head up. 

**“Levi? Mikasa said you wanted to see me, but she was acting really weird! She kept smiling at me-”** Eren cut himself off all of the sudden, he has caught Albus’s scent. Before Eren could let out a growl I spoke up. 

**“I know you’ve told me that you’re okay with us not being able to have a pup, that I’m worth it. But I see the way you look at Sky and I know you desperately want a pup that we can call our own.”** I bark softly before turning around to face my mate with a tense Albus in my arms. Eren’s eyes are blown wide as he zeroes in on the albino pup peeking out at him.  **“Eren, this is Albus. He’s like you a siren who refused to eat a human and I’ve decided that he’s going to be our pup now.”** I tell my mate proudly nuzzling the top of Albus’s head. Eren is completely frozen in spot as he looks from the pup and to me, back and forth. Eventually, Eren slowly makes his way towards us moving down to Albus’s level. 

**“Hey Albus, I’m Eren.”** He whispers softly making me smile. Albus’s eyes widened before he practically launches himself at Eren. My mate grunts at the impact, instantly wrapping his arms around the pup. Eren moves up to face me with a huge grin on his face as he nuzzles the pup just like I had. I lean forward and press a kiss to Eren’s lips which he returns with so much love I could cry. When we pull away we’re greeted by a beaming smile from the albino pup. We both smiled and leaned in to nuzzle the pup, purrs vibrating from our chests as we express our already found love for the pup. 

* * *

Albus fit in the pack just as easily as I did. Little Sky I believe was the most excited because she now had a friend to play with. Everyone loved Albus and they all made sure he knew that he was loved deeply. Specially Eren, he was an amazing father like I always knew he’d be. I think it helped that Albus was also born a siren, it gave them a very strong bond that nothing could break. Albus and I also had a very strong bond that I couldn’t even explain, not that I needed to because I loved that pup more than anything. The sirens never came looking for the pup knowing full well that Eren would rip them apart if they tried, I’m sure Zeke warned them. So now I can finally say that my life is completely perfect and it’s all because of our little albino pup, Albus. 

**Author's Note:**

> If you would like to see Albus as a teen/young adult please check out my art Instagram!
> 
> Thank you for reading, and please leave some feedback it's very appreciated!  
> Instagrams: (cosplay) wolfish.160 & (art) wolfish.draws


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